Conventional autopilots automatically steer a ship carrying the device in a manner which causes a bearing deviation of a current course of the ship from a target course, which is set manually, to be “0”. In this case, when an initial bearing deviation is large, an order rudder angle having a large value is output. Therefore, a limiter is provided so that, when the value is larger than or equal to a predetermined value, a load larger than or equal to a predetermined value is prevented from being applied to actual steering of the ship.
An output of the limiter is transferred to an actuator, which in turn outputs a response rudder angle. After the response rudder angle is added with a disturbance factor, the result is transferred to a rudder of a ship body. A motion of a ship is measured as a bearing angle using a bearing sensor.
Conventional autopilots perform turning in accordance with a limit value of the limiter when a target course having a large bearing deviation is set. Therefore, after changing a course, stationary deviation and overshoot occur, so that smooth automatic course change cannot be achieved.
This causes a significant problem that, when there is a sea area which should be avoided, such as a floating obstruction or the like, a path different from an initially predicted turning path may be actually passed, so that safe course change cannot be achieved. Therefore, in such a sea area, course change needs to be performed by an effort made by a steersman, but not using an autopilot, resulting in an increase in load to the steersman.
A conventional autopilot which solves the problem will be described.
According to the conventional autopilot, a turning radius and a turning center as well as a target course are preset, as accumulated information, on plane coordinate axes. A rudder angle is adjusted so that the track of a ship carrying the device draws an arc having the turning radius with respect to the turning center.
Thereby, automatic course change can be smoothly and stably achieved, and a course after course change can be accurately predicted. In addition, a waste motion during automatic course change is reduced, thereby reducing fuel consumption (see Patent Document 1).
Patent Document 1: JP H08-119197A
However, since the conventional autopilot does not have means for finding a position of a ship carrying the device, the rudder angle is unavoidably set to be constant when a turning motion is performed. In the case of this method, the ship is drifted by an extraneous factor, such as tidal current, wind, or the like, in a direction of the extraneous factor, so that the turning center is also moved while the ship is turning (see FIG. 1). Therefore, although a turning operation is tried by the conventional autopilot, the ship only continues to turn in a constant direction, and cannot turn around a fixed point.
An object of the present invention is to provide an automatic steering control apparatus and an autopilot which have means for turning around a desired turning center position without being affected by extraneous factors, such as tidal current and the like.